Large increases in the electrical power demand placed upon an electrical power distribution grid will tend to reduce the electrical operational frequency of the grid, causing an “under-frequency” event. For example, a heavy or sudden electrical demand may cause a particular power distribution grid having a nominal operational frequency of 50 Hz to momentarily operate at 49 Hz. An under-frequency event may last from several seconds to several minutes and are generally short in duration.
In conventional electrical power generation systems that utilize one or more heavy-duty industrial gas turbine for supplying electrical power to the grid, the rotational speed of each turbine supplying power to the grid is synchronized to the electrical frequency of the grid. As the rotational speed of a gas turbine decreases with other things being equal, its power output correspondingly decreases. Consequently, during an under-frequency event, a gas turbine will tend to output at a lower power.
Grid code regulations typically require that power generation equipment have the capability to maintain load during under-frequency events. Typically, in response to a power grid under-frequency event, gas turbine operators meet these requirements by increasing firing temperature of the gas turbine to maintain generator output within requirements. Increases in firing temperature increase power output at a given pressure ratio, which works adequately when the gas turbine does not approach any operating limits such as maximum pressure ratio capability or maximum inlet guide vane (IGV) position.
A firing temperature increase is typically achieved by an increase the fuel flow supplied to the combustor. All things otherwise equal, the increase in fuel flow results in a higher pressure at the turbine inlet, which in turn applies backpressure on the compressor. Eventually, adding more flow results in a compressor pressure limit, which typically is observed by limiting the flow through the turbine through the diversion of compressor discharge air to inlet (inlet bleed heating) and/or reduction of fuel flow (and consequently firing temperature). However, this method has limited capability to meet grid code requirements for cool ambient conditions and/or low Btu fuels (e.g. syngas) applications, due to operability limits encountered by the gas turbine compressor.
Some conventional power generating gas turbines incorporate variable inlet guide vanes (IGV). Such variable guide vanes provide the ability to adjust compressor airflow by changing incidence angle (i.e., the difference between the air angle and the mean line angle at the compressor blade leading edge) in the front stages of the compressor. These variable IGVs permit an acceptable compressor surge-free operation margin to be maintained. Typically, maintaining surge-free operation is a vital operational criterion of the compressor component for gas turbines. However, not all gas turbines are equipped with IGVs to permit employing such a technique. Further, this action alone may not be sufficient if the maximum vane position is reached and a pressure ratio limit is encountered simultaneously while attempting to increase output. In this situation, other action must be taken to alleviate the pressure limit.
Therefore a system for temporarily augmenting power of the gas turbine during an under-frequency event to provide improved grid code compliance would be desirable.